Synthetic polymers are increasingly replacing conventional materials like metal, wood, glass and paper because of their excellent mechanical properties, as well as chemical and weather resistance. However, these very properties make such polymers nonbiodegradable, and as a result, a large amount of plastic waste is accumulating in landfills, causing severe pollution. The burden of accumulating plastic waste has led to a growing interest in the development of biodegradable plastics. The environmental movement in advanced countries generally supports the idea that plastics used in packaging and other discardable containers should be biodegradable. For many biomedical, agricultural, and ecological uses, it is preferable to have a biodegradable polymer that will undergo degradation in the physiological environment or by microbial action in the soil.
Although the use of petroleum-based plastics continues to grow, limited oil reserves and the effect of plastics on the environment has generated a need for degradable plastics based on renewable sources like cereal grains or cellulose, since most synthetic plastics are resistant to microbiological attack. Many plastic articles contain biopolymers including starch or cellulose based additives such as fillers, extenders and reinforcing agents. However, the amount of biopolymer currently being used in plastics is relatively small and would account for a minor fraction of a percent of the total plastics produced.
Most synthetic (petroleum-based) polymers and natural polymers such as polysaccharides and proteins are immiscible at the molecular level. Prior researchers have used graft copolymers of starch and vinyl monomers to develop biodegradable blends. In this type of reaction, the vinyl monomer is grafted onto the starch backbone. The most common method of grafting is the free radical initiation technique, including initiation by chemical methods and initiation by irradiation. The addition of block copolymers as a compatibilizer has also been utilized, however, block copolymers in general are expensive and those from polysaccharides are difficult to synthesize.
Petroleum-based plastics can be made biodegradable by the incorporation of some carbohydrates. For example, Griffin, U.S. Pat. No. 4,016,117, discloses a biodegradable composition including a synthetic resin, a biodegradable granular filler such as starch, and a substance which is auto-oxidizable to yield a peroxide which attacks the carbon linkages in the resin. Otey, et al., U.S. Pat. No. 4,133,784 discloses biodegradable film compositions prepared from starch and ethylene acrylic acid copolymers. Brockway et al., U.S. Pat. No. 3,095,391 discloses a process to graft vinyl monomers like styrene onto starch using redox systems. Reyes, et al., U.S. Pat. No. 3,518,176 discloses a process for grafting a vinyl monomer onto starch in which the reaction is catalyzed by radiation or ceric ions.
In other processes of combining natural and synthetic polymers, the components are treated severely to produce various compositions. For example, Chinnaswami, PCT Publication No. WO 91/02757 discloses biodegradable polymers which are prepared by treating biodegradable materials such as starches and petroleum-based plastics with heat, pressure, and reagents, resulting in oxidative degradation of the polymers to small fragments of polymers which can react to form graft and block copolymers. Lay, et al., U.S. Pat. No. 5,095,054 discloses a polymer composition containing destructurized starch and at least one thermoplastic polymer which can have functional groups which is prepared by mixing the components under heat and pressure to produce a homogeneous melt mixture.
The technique of blending incompatible well-known synthetic polymers to produce "new" polymers has grown in importance in the synthetic polymer industry. Such methods have been increasingly used to obtain products of any wide range of properties rather economically. The performance of polymer blends is dependent on the interfacial interactions and the size and shape of the phases. There are two major methods of achieving interfacial control. The first is the addition of a copolymer in the blend mixture. The compatibilizing action of block copolymers is similar to the emulsifying effect of surfactants in oil and water mixtures. Reactive blending using various reacting groups is the second method of obtaining interfacial control. The final properties of the blend depend on the morphology, extent of reaction, and interfacial characteristics in the blend.
Biopolymers are difficult to mold in their natural form. For example, products made totally from starch are brittle and inflexible and thus unsuitable for most purposes. Biopolymers and plastics are incompatible when blended so that they do not mix easily and new materials prepared from these two incompatible polymers result in products that display reduced physical properties. This is due to a high interfacial tension and poor adhesion between starch and plastic phases. The high interfacial tension contributes, along with high viscosities, to the inherent difficulty of imparting the desired degree of dispersion to random mixtures. This leads to their subsequent lack of stability, to gross separation, or to stratification during later processing or use. Poor adhesion leads, in part, to very weak and brittle mechanical behavior.
Therefore, there is a need for an improved biodegradable polymer material which combines the desirable properties of plastics with significant biodegradability such that a product buried in the soil will degrade to destruction. Products in accordance with the present invention will make a significant contribution to the environment by reducing the disposal problem caused by chemically inert and bulk plastics.